Immersion heater

ABSTRACT

An improved immersion heater assembly for heating baths containing molten aluminum or magnesium and salt flux features an outer refractory jacket and an inner metal liner. The improved assembly offers resistance to both the salt and the corrosive aluminum providing extended heater life.

United States Patent 91 Parkhill et al.

3,724,447 Apr. 3, 1973 FOREIGN PATENTS OR APPLICATIONS 1,357 3/1916 Great Britain ..126/3 60 R 1,070,849 6/1967 Great Britain ..2l9/322 Primary Examiner-William F. ODea Assistant Examiner-William C. Anderson Attorney-Carl R. Lippert [57] ABSTRACT An improved immersion heater assembly for heating baths containing molten aluminum or magnesium and salt flux features an outer refractory jacket and an inner metal liner. The improved assembly offers resistance to both the salt and the corrosive aluminum providing extended heater life.

9 Claims, 1 Drawing Figure IMMERSION HEATER BACKGROUND OF THE INVENTION In the heating and treatment of molten aluminum immersion heaters are often used which, for the most part, involves no problem since electrical or combustive heaters can be employed and protected from the molten aluminum by housing in suitable refractory materials. However, when the molten metal is heated in contact with a molten salt flux of the type sometimes employed in the treatment of molten aluminum a problem presents itself in that the salt can diffuse through the refractory housing into the combustion zone where it is hydrolized by the moist combustion gases and breaks down to form toxic halogen gases since the salts employed in fluxing aluminum inevitably contain halides, specifically chlorides. Also, the salt forms a viscous taffy-like substance which accumulates within the tube until the burner port is clogged. The material is brittle and glass-like on cooling and typically contains sodium, potassium and silicon, the metals coming from the salts and the silicon from the refractory, all in the form of a complex oxide resulting from reaction with the water of combustion. The result is that the heating has to be terminated at the affected heater site and sometimes that the entire metal treatment operation has to be interrupted for heater replacement.

The reason for the leakage is unclear. Generally there is no evidence of any crack or physical failure of the refractory tube. Nonetheless the salt is visibly present inside the tube. It appears that vapor diffusion through the tube may occur and the most careful choice of refractories as to immunity or resistance to salt attack has been completely futile. For instance, a very good quality and highly regarded refractory containing silicon carbide bonded with silicon oxynitride and sold commercially under the trade name Crystolon 63 by the Norton Co. is employed as heater housing material in the treatment of molten aluminum at temperatures of l,400 F with a salt flux containing potassium and sodium chlorides together with magnesium chloride. Repeatedly the treatment has to be interrupted because of salt leaks into the heater tube.

DETAILED DESCRIPTION In the description reference is made to the FIGURE which is a schematic elevation partially in cross section of an embodiment of the invention.

Referring now to the FIGURE the present invention contemplates a heater assembly immersed in a bath 40 containing molten aluminum 44 and a salt layer 46. The side walls of the vessel containing the melt are not shown although the bottom portion 50 and top portion 54 are shown for purposes of clarity. Heater assembly 10 can be supported as shown by the top or lid portion 54. Turning now to the heater assembly 10 shown in the FIGURE, it features a more or less conventional combustive burner assembly 12 having air and fuel inlets, respectively, 14 and 16, and a burner outlet tube 18 discharging into the immersion tube portion 20. The immersion tube portion 20 includes an outer refractory jacket or shell 22 and an inner metallic liner 24. The metallic liner includes some provision to exhaust combusted gases such as vent 30 or an annular space between the burner tube 18 and liner 24. The metal liner is shown spaced from the inside bottom of the refractory jacket 22 by a plug member 32.

The outer refractory jacket 22 must be in a refractory which is completely impervious to molten aluminum at the temperatures involved in the treatment. For instance, in the contemplated process where molten aluminum or its alloys are treated at temperatures ranging from 1,l501,600 F the refractories listed below are quite suitable although Kellogg N or Crystolon 63 are preferable mainly because of availability in proper length and diameter. In general refractories based on silicon carbide bonded with a nitride of silicon provide adequate and economical performance.

YP Manufacturer Trade Name Silicon carbide Carborundum Refrax 20 or silicon nitride bond Company 50 silicon carbide with Norton Company Crystolon 63 silicon oxynitride bond silicon carbide- Norton Company Cryston silicon oxynitride composite recrystallized pure Norton Company Crystar silicon carbide high silica Thermal American Quartz Fused Quartz Company silicon carbide with Electro Kellog N silica-silicon Refractories and nitride bond Abrasives Company Notwithstanding the particular choice of any known refractory some amount of penetration by the molten reactive halide salts normally used in treating aluminum will be encountered. These salts normally include some amount of potassium or sodium chloride or both, sometimes along with a small amount of fluoride. Typical salt compositions are 60 percent NaCl-40 percent KCl; 57 percent NaCl-38 percent KCl-S percent MgCl and percent MgCl -20 percent KCl.

Because of the penetration of the refractory by the salt it is important that metallic liner 24 be selected in a metal which is sufficiently resistant to the molten salt or its vapors at the temperatures involved. Suitable metals include nickel base alloys containing at least 60 percent nickel and at least 10 percent chromium along with up to 10 percent iron, up to 3 percent manganese, up to 3 percent titanium, up to 6 percent aluminum and up to 6 percent columbium. These alloys are available from the International Nickel Co. under the name Inconel. Other suitable metals for the liner include those austinitic stainless steels known as the 18-8 series and containing 16 to 20 percent chromium and 8 to 15 percent nickel. However, the stainless steels need to be treated to withstand the temperatures of over 800 C encountered in heaters of the type here concerned. Hence the stainless steel tubes can be treated at elevated temperature in contact with powdered aluminum to cause the surface portions of the tube to be alloyed with aluminum by diffusion. The diffusion aluminum alloying effect penetrates the outer 0.005 inch or so (normally under 0.015 inch) of the stainless steel tube but this is sufficient to improve its temperature resistance to an acceptable level. One such process found to be suitable is known as Alonizing and is available from Alon Processing, Inc. of Tarentum, Pa. Other metals are also suitable, the principal requisites being resistance to temperature and corrosion by molten halide salts and their vapors. However, the stainless steel and Inconel alloys are preferred from the standpoint of economies.

It is not essential that the metal liner 24 fit tightly against the inside wall of the outer refractory jacket 22. On the contrary, as shown in the FIGURE, there may be situated a gap 28 of significant size. For instance, an annular gap 28 can vary from 1/16 to inch without drastically curtailing heat transfer. In addition, some gap may be desirable because of differential thermal expansion. It is also desirable that an outlet 36 be provided to vent or provide some escape for any salt fumes or the like which permeate through the refractory jacket 22 so as to allow an easy escape thereof from gap 28. The end spacer 32 is advisable since it prevents radial expansion of the bottom of liner 24 from applying force against the beveled inside portion of the bottom of the refractory jacket 22. Sufficient force here could cause the jacket to crack.

Thus the invention contemplates the use of an outer refractory shell or jacket impervious to molten aluminum or magnesium and an inner metal liner impervious to the molten salt. As is known there are practically no metals which are completely immune to attack from molten aluminum at temperatures over l,200 F. For instance, while Inconel or stainless steel of the types described above suitably serve as the liner for the improved immersion heater assembly, such could not serve without the outer refractory jacket since they would be repeatedly corroded by the molten aluminum and contaminate the melt with corrosion products. Similarly, but to a lesser extent, even titanium is corroded by molten aluminum so there is little if anything to offset its higher cost. In one sense, then, the inven-' tion contemplates a cooperative relationship among the elements of the improved heater tube assembly in resisting attack in molten aluminum-salt flux baths. The outer refractory shell or jacket is not attacked or penetrated by the molten metal but is permeated through by the molten salt flux. The inner metal liner, while not resistant to the molten metal, is, however, impenetrable by the salt flux which permeates through the outer shell.

The present improvement has been employed in numerous arrangements and systems for the heating of molten baths containing aluminum and molten halide salts. Notwithstanding extended heating time periods the heaters have repeatedly withstood the extremely corrosive environment presented by the bath. Typically the use of the improvement has extended burner life from about four days to over one hundred days, an increase of 25 fold.

The invention has been described with particular reference to molten aluminum and its treatment. However, it is believed that the improved heater should also be useful in the treatment of magnesium. Hence the term light metal is employed to encompass both aluminum and magnesium together with their alloys. Also, while the invention has been described as suited to use with combustion burners, it is also useful with electrical or other heat sources.

We claim: g

1. An improved immersion heater comprising an outer refractory shell, an inner metallic liner, spaced from said shell to provide an annular gap therebetween, and a heat source arranged to apply heat within said metallic liner for transmission therethrough and throu hsaid refracto shell,

2. In a system 0 treating a molten light metal selected from the group consisting of aluminum or magnesium or their alloys in the presence of a reactive molten halide salt flux the steps comprising applying heat to said bath and the metal and salt container therewithin through an immersion heater comprising an outer refractory shell impervious to said molten metal but penetrable by said salt flux and a metal liner spaced within said jacket not corrodible by said salt and a heat source to provide heat within said metal liner for transmission to said bath.

3. The immersion heater according to claim 1 wherein said metallic liner is composed of a nickel-base alloy containing at least 60 percent Ni and at least 10 percent Cr.

4. The improved heater according to claim 1 wherein the refractory shell is composed of silicon carbide bonded with a nitride of silicon. i

5. The improved heater according to claim 1 wherein said heat source is a combustion burner.

6. The improved heater according to claim 1 wherein said metallic liner is composed of an austinitic stainless steel of the 18-8 type which has its surfaces aluminum diffusion alloyed to improve temperature resistance.

7. The improved system according to claim 2 wherein the molten light metal'is aluminum or an alloy thereof and the halide salt flux includes alkaline or a]- kaline earth chlorides.

8. The improved system according I to claim 7 wherein the refractory shell is composed of silicon carbide bonded with a nitride of silicon, and the metal liner is composed of a nickel-base alloy containing at least 60 percent Ni and at least 10 percent Cr.

9. The improved system according to claim 7 wherein the refractory shell is composed of silicon car-.

bide bonded with a nitride of silicon and the metal liner is composed of an austinitic stainlesssteel of the 18-8 type which has its surfaces aluminum diffusion alloyed to improve temperature resistance. 

2. In a system of treating a molten light metal selected from the group consisting of aluminum or magnesium or their alloys in the presence of a reactive molten halide salt flux the steps comprising applying heat to said bath and the metal and salt container therewithin through an immersion heater comprising an outer refractory shell impervious to said molten metal but penetrable by said salt flux and a metal liner spaced within said jacket not corrodible by said salt and a heat source to provide heat within said metal liner for transmission to said bath.
 3. The immersion heater according to claim 1 wherein said metallic liner is composed of a nickel-base alloy containing at least 60 percent Ni and at least 10 percent Cr.
 4. The improved heater according to claim 1 wherein the refractory shell is composed of silicOn carbide bonded with a nitride of silicon.
 5. The improved heater according to claim 1 wherein said heat source is a combustion burner.
 6. The improved heater according to claim 1 wherein said metallic liner is composed of an austinitic stainless steel of the 18-8 type which has its surfaces aluminum diffusion alloyed to improve temperature resistance.
 7. The improved system according to claim 2 wherein the molten light metal is aluminum or an alloy thereof and the halide salt flux includes alkaline or alkaline earth chlorides.
 8. The improved system according to claim 7 wherein the refractory shell is composed of silicon carbide bonded with a nitride of silicon, and the metal liner is composed of a nickel-base alloy containing at least 60 percent Ni and at least 10 percent Cr.
 9. The improved system according to claim 7 wherein the refractory shell is composed of silicon carbide bonded with a nitride of silicon and the metal liner is composed of an austinitic stainless steel of the 18-8 type which has its surfaces aluminum diffusion alloyed to improve temperature resistance. 